If you see one of these messages, it often indicates that you’ve tried to load a key of an inappropriate type into WinSCP.

You may have specified a key that’s inappropriate for the connection you’re making. The SSH-1 and SSH-2 protocols require different private key formats, and a SSH-1 key can’t be used for a SSH-2 connection (or vice versa).

Alternatively, you may have tried to load an SSH-2 key in a “foreign” format (OpenSSH or ssh.com), in which case you need to import it into PuTTY's native format.1)